The present invention relates in general to an improved feedwater control system for drum type steam generators, and in particular to a new and useful method and apparatus for providing improved drum level control to units which are subjected to great load changes at high rates of change.
The current feedwater control systems for drum type steam generators include the popular three-element feedwater control system which is a cascade-feedforward control loop which maintains water flow input equal to feedwater demand. The system employs feedback of the difference between the desired level as indicated by the set point and the actual level to compensate for any system errors such as transmitter drift, flow measurement errors etc.
A typical prior art feedwater control system is illustrated in FIG. 1.
A feedwater control valve 180 controls the sending of feedwater 200 to a drum type steam generator through three separate controls, namely a drum level transmitter 110, a steam flow transmitter 120, and a feedwater flow transmitter 140.
The drum level transmitter 110 sends a drum level signal 112 to a difference unit 116 wherein the difference between the drum level signal 112 and a drum level set point 105, originating from an analog control 170 is formed. The difference resulting from the comparison of the drum level signal 112 and the drum level set point 105 is the drum level error 108.
A drum level indicator 114 displays the value of the drum level signal 112 sent by the drum level transmitter 110.
The transmitter 120 sends a differential pressure signal 121 to a square root unit 122 for taking the square root of the signal 121 to determine the steam flow value 123.
A steam flow indicator 124 displays the steam flow value 123 after execution of the square root function by the square root unit 122.
The steam flow value 123 is then sent to a summation unit 128 which is a proportion controller which performs a summing function of the drum level error 108 and the steam flow value 123 in order to determine a summation value 130.
The differential pressure transmitter 140 sends a pressure signal 141 to a feedwater flow square root unit 142 which performs a square root function on the signal 141. A feedwater flow indicator 144 displays the feedwater flow signal 143.
The feedwater flow value 143 is sent to a signal lag unit 148 which delays the sending of the feedwater flow value 143 for a period of time determined by the function f(t) resulting in the signal lag value 150.
A PI controller 160 serves as a proportional action-plus integral controller for the deviation between the output of the summer unit 128 and the output of the signal lag unit 148 (where .DELTA. is the deviation, K is proportion and .intg. is the integral) and determines the output value 165 of the PI controller.
Once determined, the PI value 165 is then sent to the hand/auto station 170 which serves as the automatic control station with bias for the system. The station control 170, based on the PI value 165, will automatically control a feedwater control valve 180 which adjusts the amount of feedwater 200 sent to the drum type steam generator.
Although a three element feedwater control system can provide satisfactory feedwater control when the boiler is operated at steady loads through the use of the drum level set point 105 specified in FIG. 1, it is incapable of handling great load changes at high rates of change because there is not a way for the system to account for the change in mass inventory within the circulation loop. The circulation loop existing as the water and water/steam circulation system comprises the steam drum water space, the downcomers, the supplies, the furnace water walls, the boiler bank and mud drum, risers and the water/steam annulus in the steam drum.
The current state of the art in feedwater control systems can only react to a change in mass inventory once the drum level diverges from the set point. Thus, during a load change, the drum level cannot be maintained.